The present invention relates to a semiconductor device with a semiconductor element and a composite lead wire or an electrode attached to the semiconductor element.
Many types of semiconductor devices, such as diodes, transistors, IC chips, LSI chips, laser diodes, photovoltaic generating devices and the like are widely used for various fields because of their compact size and their high efficiencies.
Referring now to FIGS. 1 and 2, as one example of the prior art semiconductor devices, a photovoltaic generating device, a so called solar battery cell, will be explained. A shallow n+ layer 1 is formed by diffusion or ion implantation in the upper surface of a p-silicon substrate 3. An n+-p junction 4 is thereby formed less than 1 .mu.m from the surface. On the entire rear surface of substrate 3, a back electrode 5 is formed on a p+ layer 6. Back electrode 5 is formed by deposition or plating of aluminum or silver. On the front, surface of the substrate, that is on n+ layer 1, fine collecting electrodes defining a grid 7 for collecting photovoltaic current and main collecting electrodes defining bus lines 8 are provided. Both grid 7 and bus lines 8 are formed by plating, depositing, or sintering. Bus lines 8 are formed thicker than grid 7. Both are formed so that the open area is 90% to 99% of the entire surface. The end 9 of bus line 8 is formed so as to be able to be connected to another photovoltaic generating element via a lead wire. On the entire surfaces of n+ layer 1, grid 7 and bus lines 8, an antireflection film 10 is formed so as to convert incident light 12 into electric power with high efficiency.
In a photovoltaic generating device as described above, grid 7 and bus lines 8 are made of highly electrically conductive material, for example silver. However, since the open space must be about 90% to 99%, as a result grid 7 and bus lines 8 have to be formed narrow. Therefore, grid 7 and bus lines 8 have to be thick to minimize electric resistance. However, a photovoltaic generating device having thickened bus lines 8, for example of thickness more than about 15 .mu.m, tends to warp or crack since the difference in thermal expansion coefficient between the bus lines and a photovoltaic generating element made of silicon is relatively large. This occurs during the heat step of manufacturing or during the time the device is operating in the sunshine.
Accordingly, several proposals have been made for removing or reducing the above-described defect. Referring now to FIG. 3, circular grids 15 and radial grids 16 are provided instead of bus lines. The generated currents are collected at a center terminal 17 through radial grids 16, and the collected current is taken out by a mesh lead wire 19, which is connected to center terminal 17 and is made of, for example, silver. According to this structure, the mesh lead wire 19 is fixed to only one point at center terminal 17, so that the position of lead wire 19 is flexible. Thus, measuring the characteristics and inserting the device into a tool for the next step in manufacturing is inconvenient. This approach has the additional drawback of decreasing the photovoltaic generating ability because of the shading by lead wire 19.
Referring now to FIG. 4, in this device a bus line is not provided. Instead of the bus line, thin copper lead wires 27 of about 100 .mu.m thickness are used. Bonding pads 22 connected to grid 26 are provided and copper lead wires 27 are fixed to bonding pads 22 by soldering. According to this device, the electric resistance can be decreased by using copper lead wire. The problem caused by the difference of the thermal expansion coefficient between copper lead wire 27 and photovoltaic generating element 28 made of Si can be avoided by fixing copper lead wire 27 with local bonding pads 22 with small area. However, when this device is attached to a protective glass panel by thermal press fitting using an organic sheet, thermal distortions are concentrated on bonding pads 22. Such thermal distortions may break the photovoltaic generating element or cause thermal fatigue. Furthermore a gap can be easily formed between the copper lead wire and the photovoltaic generating element. As a result, bubbles are produced under the organic sheet. Also the copper lead wires can contact the surface of the photovoltaic generating element, with the result that the surface of the photovoltaic element will be rubbed and the characteristics will deteriorate.
A semiconductor device handling large current, such as a thyristor, a transistor, a diode, etc., includes a stem and a semiconductor element mounted on the stem by soldering. The stem is made of highly electrically and thermally counductive material, for example copper. A several times differential exists between the thermal expansion of the semiconductor element and that of the stem. This semiconductor device is exposed to a heat cycle during manufacturing and during operation. The heat cycle causes thermal fatigue which can result in peeling of the solder, and deterioration and break of the semiconductor element. If a semiconductor element is made of silicon, generally a tungsten or molybdenum electrode is provided as a buffer between the semiconductor element and the stem. Tungsten and molybdenum have approximately the same thermal expansion coefficient as silicon. However, tungsten and molybdenum are high in price and their electrical conductivities are about three times lower than that of copper. Because of the resistive heat produced by low conductivity, the current capacity is limited to a low level so as to keep the temperature below the permitted maximum value of the junction temperature, for example 150.degree. C. in case of silicon semiconductor devices. The above-described factors are also applicable to a laser diode (a light emitting diode) using GaAs as a semiconductor element.
Furthermore an IC or an LSI device has a lead frame for mounting the chip. When the lead frame is made of copper of 250 .mu.m in thickness, IC or LSI chips of size more than about 3.5 mm.times.3.5 mm may be broken because of the differential of the thermal expansion coefficient between the lead frame and the IC or the LSI chip. Therefore, in case of a relatively large size LSI chip, for example 8 mm.times.8 mm, the lead frame is made of alloy 42%Ni-Fe. However this alloy has one order of magnitude larger electrical resistance than copper and also has a larger thermal resistance of about 100.degree. C./w compared with copper of about 40.degree. C./w. Consequently, the current capacity must be reduced to keep the temperature below the permitted maximum value of the junction temperature.